Method of forming low flux loss bonds in permanent magnet assemblies



Aug. 1, 1961 sFzdM ERKER ETAL P MANENT MAGNET ASSEMBLIES Original FiledOct. 19, 1955 METHOD FlG.l

LOW FLUX LOSS BONDS I INVENTORS SIEGFRIED ZUERKER, BENJAMIN J. SAGGESE,

THEIR ATTORNEY.

United States Fatent g 2,994,122 METHOD OF FORMING LOW FLUX LOSS BONDSIN PERMANENT MAGNET ASSEMBLIES Siegfried J. Zuerker, Utica, and BenjaminJ. Sa'ggese,

Liverpool, N.Y., assignors to General Electric Company, a corporation ofNew York Original application Oct. 19, 1955, Ser. No. 541,436, nowPatent No. 2,894,182, dated July 7, 1959. Divided and this applicationMar. 9, 1959, Ser. No. 797,964

2 Claims. (Cl. 29-15556) This application is a division of co-pendingapplication for Electro-Acoustic Transducer, Serial Number 541,436,filed October 19, 1955, issued July 7, 1959, as US. Patent No. 2,894,182which is assigned to the same assignee as the present application.

This invention relates to an improved structure for the magneticcomponent of a transducer device.

It is an object of this invention to provide a novel magnetic structurefor a transducer, such as a loudspeaker which permits fabrication at areduced cost.

It is another object of this invention to provide a novel magneticstructure for loudspeakers, which structure has a configurationpermitting it to be molded out of a powdered magnetic material.

It is still another object of this invention to provide a novel magneticstructure for loudspeakers, wherein the magnetic structure may beassembled to the rest of the speaker assembly by the use of a portion ofthe magnetic structure itself so that the cost of assembly is reduced.

In the prior art wherein one component of magnetic material has beensoldered to another, it has been found necessary to first coat thecomponents with a suitable base before applying the solder. This methodof bonding results in a relatively thick eflective air gap between thetwo members and a consequent increase of reluctance in the circuit.Therefore, it is a further object of this invention to provide a novelmethod of bonding magnetic materials so that the eifective air gapbetween the materials is minimized.

It is still a further object of this invention to provide a novel meansand method of bonding components of magnetic structures to one anotherso that the cost of assembly of these components is reduced and thesensitivity of the structures are increased.

Briefly, the objects of this invention are achieved in one form by theprovision of a unitary structure forming a portionof the magneticcircuit of a loudspeaker, which unitary structure has provided thereonan integral means supporting the magnetic structure on the speakerbasket. A second portion completes the magnetic circuit and is assembledto the remainder of the magnetic structure by the use of a bondingmaterial which reduces the equivalent air gap normally existing betweenthe second portion and the rest of the magnetic structure and therebycuts down the consequent flux loss and yields a more sensitiveinstrument.

The novel features which we believe to be characteristic of ourinvention are set forth with particularity in the appended claims. Ourinvention itself, however, both as to its structure and method ofassembly, will best be understood by reference to the followingdescription taken in connection with the accompanying drawing, in which:

FIGURE 1 is a cross-sectional side view of a speaker embodying ourinvention; and

FIGURE 2 is a rear view of the speaker illustrated in FIGURE 1.

Referring now to FIGURE 1 of the drawing, it may be seen that thespeaker is constituted by the supporting frame or basket 1. At the outerperiphery of the basket a flange 2 supports the outer edge of afrusto-conical known in the art. Also, in the manner well known the art,we provide a flexible spider 6, which is secured at its outer peripheryto the basket as shown at 7. A moving or voice coil 8 is wound on thevoice coil form 5 and drives the diaphragm 3 in accordance with thevarying electrical signals representing sound intelligence impressedthereon. The construction described is substantially that constructionwell known in the art and constitutes no part of our invention.

In accordance with our invention, we provide aper-,

. manent magnet '9 which may have a cylindrical configuration and isdisposed, as illustrated in the drawing, just to the rear of thebasket 1. The magnet may, if desired, be constituted by an electromagnetor may be a permanent magnet formed from the material known by the tradename Alnico V which is commonly used in the 1 art for this purpose. Apole plate 10 of magnetic material may be secured by any suitable meansto the front of the magnet 9 or may be held by the action of the magnetitself and provides a path to carry magnetic flux to a regionsurrounding the voice coil 8. Alternatively the pole plate 10 may beeliminated and the magnet9 extended to the region occupied by the poleplate. A

cup or yoke structure 11, which is formed as a tube and which may have acircular cross-section, encompasses the magnet 9 and also constitutes aportion of the magnetic circuit of the device. The remainder of themagnetic circuit is completed by the bottom plate 12 which may be formedfrom a soft steel or other magnetic mate- 1 rial and is secured to therear end of the magnet 9 and engages the inner surface of the largerinner diameter end of the cup. A forward portion of the cup 11 isprovided with a portion extending radially inward as shown at 13 andpresents a smaller inner diameter. In combination with the pole plate10, the portion -13 forms an air gap 1 4 in which the voice coil 8 issituated, whereby the electrical conductors constituting this coil maybe transversed by the magnetic flux emanating from the magnet 9. Theextent of the inwardly directed portion is selected inaccordance withthe principle well known in the art so that the required spacing betweenthe outer periphery of the magnet 9 and the inner wall of the cup 11 isat an optimum to maintain fringe flux losses between these two membersat a The forward end of the portion 13 is provided with a means wherebythe cup may be secured to the basket 1 by positioning extension 13, inan opening in the rear portion 15 of the basket 1 and peening it over toengage the frame in the manner indicated by reference numeral 16.

The construction of the magnetic portion of the speaker is such that theyoke or cup 13 may be molded using any suitable material capable ofcarrying magnetic flux. Thus, it is possible to utilize a powdered ironmaterial for this portion of the assembly which may be fabricated bymolding so as to considerably reduce the cost of this portion of thedevice. This particular advantage is gained by providing that theforward reduced inner diameter portion 13 of the cup constitutes theelement completing the air gap, necessary for the operation of the voicecoil.

As pointed out above, the magnetic circuit is completed by the bottomplate 12. The bottom plate, in addition to completing the magneticcircuit, also functions to make possible the use of material having arelatively low saturation value, and consequently relatively inexpensivematerials, as it reduces the flux density by providing Patented Aug. 1,196 1 spacing or securing ring,such

a path of ever increasing cross-section from its center to its peripherywhere it engages the cup 111. Thus, a. powdered iron material is capableof transmitting the required flux without incurring high osses due tohigh flux density.

A further feature of our invention is the manner in which the bottomplate 12 and the pole plate are secured to the permanent magnet 9. Ithas been the custorn in the prior art to solder such assemblies. Thesoldering operation has been carried out by coating the members with abase such as cadmium and by depositing pellets of solder and fluxbetween the surfaces of the two members after the coating. Heat is thenapplied and the solder disperses over the two surfaces. However, thestructure which thereby results is subject to the disadvantage of whatamounts to a fairly large gap exists between the two members 9 and 12.This air gap may be on the order of 0.002 of an inch and the resultingflux losses serve to reduce the sensitivity of the device.

In accordance with our invention, we provide that the opposing surfacesof the pole plate 10, bottom plate 12 and the magnet 9 shall each becleaned by the application of a suitable flux or a cleansing agent andthen coated with a relatively thin coat of tinzinc. It has been foundthat a tin-zinc composition of from 73%- 83% tin and from. 27%17% zinc,by weight, yields a bond of sufficient strength to hold the memberstogether. Within this range, a composition of 78% tin and 22% zinc, byweight, appears to yield a bond of optimum strength characteristics. Wehave found that electroplating the most effective method for forming acoating of tin-zinc on the surfaces to be joined. The members 9, 10 and12 are then subjected to heat. The tin- Zinc forms, upon cooling, atstrong bond to hold the members 9, 10 and 12 together. The equivalentair gap between these members is far less; than that obtained by othermethods such as noted above and the consequent losses are avoided. Theair gaps, obtainable by the use of method, are on the order of 0.0002 ofan inch and an over-all increase in speaker sensitivity is obtained.Such an advantage permits either the construction of a more sensitivespeaker or, sensitivity comparable to competing speakers is desired, asmaller magnet with its consequent cost reducing feature may be used. Wehave tound that if a fiurther decrease in the equivalent air gap isdesired, pressure may be exerted on the elements 9, 10 and 12 to forceout whatever bubbles or undue thickness of plating material may bepresent.

The bonding technique, described above, has particular utility informing structures constituting a magnetic circuit wherein it isdesirable to avoid flux losses. However, it is not limited in itsapplication to loudspeaker magnetic structures, but obviously hasgeneral utility in fabricating magnetic devices of diversecharacteristics.

While the present invention is described by reference to a particularembodiment thereof, it will be understood that numerous modificationsmay be made by those skilled in the art without actually departing fromthe invention. We therefore, in the appended claims to cover all suchvariations as come within the true spirit and scopeof the foregoingdisclosure.

What We claim as new and esire to secure by Letters Patent of the UnitedStates is:

l. A method of bonding a permanent magnet to a portion of a loudspeakermagnetic structure comprising the steps of, electroplating the magnetand the magnetic structure with tin-zinc, and subjecting them to heat tocause the tin-zinc to form a bond.

2. A method of bonding magnetic materials comprising the steps ofelectroplating a coating of tin-zinc on each surface of each member tobe bonded, placing the plated surfaces in contact with each other,subjecting the members to heat to cause the tin-zinc to form a thin lowflux loss bond.

References Cited in the file of this patent UNITED STATES PATENTS440,952 Land NOV. 18, 1890 2,169,098 Howe Aug. 8, 1939 2,798,843 Slominet a1 July 9, 1957 ,365 Erickson Feb. 25, 1958

